This project features a cast-in-place countertop with an optional terrazzo finish flecked with bits of white marble, mother-of-pearl, and cobalt-blue glass. Casting the countertop in place lets you skip both the heavy lifting and the "upside down and backward" thinking involved in precasting into a mirror-image mold. But troweling the surface smooth does require skill. Practice on other projects first.
The countertop rests on a base of ½-inch-thick cement board, enabling you to keep the concrete 1 inch thick except at the edges, which are just deep enough to hide the cement board. The result is a normal-height countertop that's easily supported by standard cabinets. Reinforcing consists of galvanized-steel stucco lath, so you must use aggregate small enough to fit through the openings. We used Buddy Rhodes Concrete Counter Mix, which contains white cement and bits of white marble, but a standard sand mix would be fine. We troweled blue recycled glass and a few handfuls of mother-of-pearl into the surface and sanded it a few days later by hand, because a polishing machine would have made a mess in the room.
1. Make a template by outlining the shape of your countertop on cardboard. The edges should be flush with the sides of the cabinet. If the counter is bigger than a sheet of cement board, mark joints on the template; they must be placed over cabinet walls so the board is supported from below. Mark the sink outline using the template packaged with it. You can also use the sink template is to create a plug out of foam insulation that's at least 1½ inches thick. This will keep the sink free of concrete. A jigsaw or bandsaw works well to cut foam. Sand edges of the foam and wrap them with plastic tape so that they are smooth. If you see gaps that you don't like, fill them with a paste of cement and water. Allow them to harden for a few days. Polish with wet-dry sandpaper. Seal and wax the counter before you use it.
2. Cut the cement board to fit. For straight cuts, score a line with a utility knife or a knife made for cement board. Snap the sides into a fold and cut through the board's interior mesh, as shown.
3. Cut a hole for the sink and other curves with a jigsaw fitted with a wood blade. The blade will wear out fast; have a spare.
4. Drape plastic over the cabinets and against the backsplash. Clamp 2 x 4s along the top of the cabinets to support the plastic molding that will form the countertop's outside edge; place wooden molding of equal height along the backsplash. Then set the cement board in place so that it butts against the backsplash molding. Secure the foam plug in place with pieces of scrap wood from below. To guard against cracks appearing in the countertop, cover seams in the cement board with the adhesive mesh tape.
5. With tin snips, cut 2½-pound galvanized-steel stucco mesh to match the countertop, minus a 1-inch gap along all edges, including around the sink. If you need multiple pieces, overlap the mesh by a few inches. Before you mix the concrete, dampen the cement board with water. If the cement board dries out, repeat this step just before you fill the form, but blot up any puddles.
6. Prepare basic sand mix (1 part cement, 2 parts sand, approximately ½ part water) or a bagged sand mix. With a putty knife, take a little of the prepared mix and press it down into seams covered with tape. By hand, pack more of the mixture into the edge form. Work quickly to fill the rest of the form halfway.
7. Place the wire mesh on top of the damp mix. With a float, lightly press the mesh into the bottom layer of mix, just enough so the wire doesn't curl. Make sure it isn't too close to the edges. Then fill the form the rest of the way. With a hammer or stick, tap along the edge band to release air bubbles. Then level the concrete with a straight board as you would for any poured project.
8. Sprinkle glass pieces, mother-of-pearl, or other inlays on the surface.
9. Embed the inlays by troweling over them. To eliminate gaps around their edges, trowel until they are completely covered with the cream of cement and fine sand that is on the surface. Once you've embedded the inlays, cover the countertop with plastic. For the next three days, periodically mist the concrete to keep it damp.
10. Remove the edge form and cut out the sink plug. Then begin polishing with wet-dry sandpaper, diamond pads, or solid aluminum-carbide blocks. Start with 100- or 120-grit abrasive and work up to 220 and then 440. If sanding ejects whole pieces of sand or other aggregate, delay this step for another day. Keep handy a pan of water and a sponge (preferably the type sold for masonry jobs) so you can keep the surface damp and periodically wipe away the slurry you will create. Stop sanding when you like the effect.